Field of the Invention
The present invention relates to an optical fiber pressure sensor and a pressure measuring method using the same, and more particularly, to an optical fiber pressure sensor capable of measuring pressure through change in wavelength of output interference spectrum at the time of applying pressure.
Description of the Related Art
Nowadays, many sensors are used to prevent a safety accident in advance at industrial sites, and new sensors have been developed up to now. Among them, pressure sensors are used in various industrial fields. When the sensors described above are embodied using optical fibers, it is possible to make the sensors smaller and lighter than electronic sensors, and there is no influence of electromagnetic interference on them. In addition, there is an advantage that optical fiber is strong against corrosion and has a life longer than that of a general electric wire.
Particularly, in order to optically measure pressure while making the best of above advantages, polarization-maintaining fiber (PMF) can be used. The PMF is an optical fiber that maintains polarization of polarized incident light to an optical fiber output terminal as it is by inducing large birefringence to an optical fiber core. As a method of inducing birefringence to an optical fiber core, there are a method of inserting a stress rod with a different expansion rate to a part of cladding to apply stress to the core, and a method of deforming a shape of a core into an elliptical shape other than a circular shape. As types of PMF described above, there are a panda type PMF, a bow-tie type PMF, and an elliptical clad type PMF. When external environmental factors such as temperature, pressure, strain, etc. around such a PMF are changed, physical properties of the PMF are varied. Using these characteristics that the physical properties of the PMF are varied, it is possible to measure change in various physical parameters applied to the PMF.
In particular, a Sagnac birefringence interferometer based on polarization-maintaining photonic crystal fiber (PM-PCF) has a simple structure, has high sensitivity of PM-PCF physical property to change in external physical parameters, has a characteristic which does not depend on input polarization, and is insensitive to change in external temperature, and thus it is widely used as a sensor head or a demodulation filter recently.
However, the PM-PCF used as the sensor head of the optical fiber pressure sensor is expensive over ten times as compared with the PMF, and has pressure sensitivity equal to or lower than the half, which are its disadvantages. In addition, the internal structure of the PM-PCF contains the periodical arrangement of air holes, and thus fusion splicing with general optical fibers is difficult, resulting in the increase of fusion splicing loss. And the increased splicing loss disturbs the accuracy of a sensor, which is another problem.
As a related report, an optical fiber pressure sensor having a Sagnac birefringence interferometer structure formed of PM-PCF with a length of 58.4 cm was demonstrated in Appl. Optics. vol. 47, pp. 2835-2839, 2008. In the previous study, a measurement range was 0 to 0.3 MPa, and measured pressure sensitivity was about 3.42 nm/MPa.